1. Technical Field
This application relates to computer storage devices, and more particularly to the field of transferring data between storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
During operation, some data for a storage system may be temporarily stored in volatile memory. For example, data that is usually on disks may temporarily be cached in volatile memory to improve performance of the system. Any disk data that is modified after being read into cache is eventually destaged (written) back to the disk. In addition, metadata and other system parameters may be stored in volatile memory. It is desirable to maintain this data if a system power failure occurs. To this end, the system may be provided with battery backup that may be used in connection with transferring data from volatile memory to non-volatile memory following a power failure.
Generally, a system may be designed so that if power fails, the battery has enough power (time) to transfer data from volatile memory to non-volatile memory. However, in some instances, a power failure may be preceded by component failures that may affect the amount of time it takes to transfer data from volatile memory to non-volatile memory. It also possible for components of the battery to fail, thus decreasing battery power and the amount of time available to transfer the data. When the system is thus degraded, it is useful for a user (operator) to know when there is no longer enough battery power to guarantee transfer of all of the data from volatile memory to non-volatile memory following a power failure. It may also be useful to take steps to decrease an amount of time needed to transfer the data. Accordingly, it is desirable to provide a system that addresses these issues.